This invention relates to a synthetic aperture radar apparatus. More particularly, it relates to a synthetic aperture radar apparatus constructed of a synthetic aperture radar device which is mounted in an artificial satellite and with which a land surface, etc. are observed, and an altitude calculation device which is mounted in the artificial satellite and which measures the acceleration of the satellite, thereby to calculate the altitude of the satellite.
FIG. 2 is a diagram showing an example of a prior-art synthetic aperture radar apparatus. In the figure, numeral 1 designates a transmitter, numeral 2 a circulator, numeral 3 an antenna, numeral 4 the earth, numeral 5 a receiver, numeral 6 a data recorder, and numeral 7 an operation controller.
A transmission pulse sent from the transmitter 1 in the synthetic aperture radar apparatus is led through the circulator 2 to the antenna 3, and it is thereafter radiated as an electric wave from the antenna 3 toward the earth 4. This electric wave is scattered in various directions on the surface of the earth 4. Some of the scattered waves are received by the antenna 3 again. The received waves, namely, observational echoes are led through the circulator 2 to the receiver 5, and they are subjected to amplification, frequency conversion and analog-to-digital conversion by the receiver 5. The digital data thus obtained is recorded by the data recorder 6.
The above operations are repeatedly performed every transmission pulse that is transmitted at a fixed period with the movement of an artificial satellite, not shown, carrying the synthetic aperture radar apparatus. Herein, all of control commands such as an observation start command for the synthetic aperture radar apparatus, an ON/OFF command for the power source of the transmitter 1, a command for setting the pulse-recurrence frequency of the transmitter 1, a command for setting the gain of the receiver 5, a record/reproduce command for the data recorder 6, and an observation end command, are delivered from the operation controller 7. Image processing is executed on the basis of the observational data recorded on the data recorder 6, whereby the image of an observed region is obtained.
Meanwhile, in the case of observing a land surface or the like with the synthetic aperture radar apparatus, the transmission pulse which is transmitted from the transmitter 1 must be transmitted at a timing at which the following two conditions are satisfied:
(1) The transmission pulse and an echo from the observed region do not overlap. PA0 (2) A nadir echo (an echo from a point lying just under the satellite) and an echo from the observed region do not overlap.
Accordingly, the pulse-recurrence frequency of the transmission pulses is set so as to meet the aforementioned conditions. However, the propagation distance of the pulse varies according to changes in a distance from the artificial satellite to the observed region, namely, the altitude of the satellite, so that the timing comes to deviate.
It is therefore necessary that several pulse-recurrence frequencies of the transmission pulses are prepared beforehand in anticipation of the fluctuating component of the altitude, and that the pulse-recurrence frequency is properly selected and set according to the satellite altitude.
Since the satellite altitude varies depending upon an observed position on the earth, a predetermined pulse-recurrence frequency has heretofore been selected and set according to the observed position on the earth as illustrated in FIG. 3 by way of example. For example, in case of observing a region which belongs to a first zone, the pulse-recurrence frequency of the transmitter 1 needs to be set at f.sub.1, and in case of observing a region which belongs to a second zone, the pulse-recurrence frequency of the transmitter 1 needs to be set at f.sub.2. The same applies to a frequency f.sub.3 corresponding to a third zone, and a frequency f.sub.4 corresponding to a fourth zone.
Accordingly, in a case where the synthetic aperture radar apparatus is mounted on a flying object such as the artificial satellite for the purpose of observing a land surface etc., the setting of the pulse-recurrence frequency of the transmitter 1 needs to be altered according to the change of an observed zone.
With the prior-art synthetic aperture radar apparatus as stated above, the orbital changes of the satellite are predicted, and the operation of altering the pulse-recurrence frequency in accordance with altitudinal changes attendant upon the orbital changes is programmed for the operation of the radar apparatus. A command for altering the pulse-recurrence frequency on the basis of the program is delivered from the operation controller 7 to the transmitter 1. With such a programmable operation based on the predicted changes, however, only the setting operations based on predictive data are possible to the last, and the setting of the pulse-recurrence frequency according to an actual altitudinal change cannot be realized.
Accordingly, an unexpected altitudinal change on the orbit of the satellite cannot be coped with, so that an accurate pulse-recurrence frequency cannot be set.